In the document scan apparatus which scans an image of a document supported on the document support table, a reference position of the document support platen relative to the optical system is detected during the relative movement of the document support platen and the optical system to control the scan operation of the document. In a typical case, the optical system, including a light source, mirror and a CCD (Charge Coupled Device) sensor array, if fixedly mounted, and the document support table is reciprocally moved relative to the optical system. Published unexamined Japanese patent application 58-124366, published on July 23, 1983 shows a slit pattern provided on a side area of a document support table. A CCD sensor array has first portion which senses the reflected light from the slit pattern, and second portion which senses the reflected light from the document image, during the movement of the table. A control unit controls the movement of the document support table in response to output signal of the CCD sensor array representing the slit pattern. The document scan apparatus in the application 58-124366 requires the above first portion of the CCD sensor array for the slit pattern, and increased width of the table, resulting in the cost up of the document scan apparatus.
To solve the problem, the inventors of the present invention had initially tried to replace the side slit mark by a reference position mark located near the top edge of a document area of the document support table. In this type of arrangement, the CCD sensor array assigned for the document image could also senses the reference position mark. In other words, the CCD sensor array senses the reference position mark at the first, then senses the document image. But, this approach had a new problem that since the documents to be scanned could include unexpected various patterns, the probability of occurrence of the same pattern in the documents as the pattern of the reference position mark could not be reduced to zero percent. If the same pattern as the reference position mark appears in the document, the case might be happen that the control unit of the document scan apparatus might erroneously determine the pattern of the document as the reference position mark, resulting in an error in the document scan. Accordingly, it has been required to distinguish the reference position mark located within the field of view of the CCD sensor array for sensing the document image from the document image.